Hardiness:I have not had any experience with this species yet. But according
to David J. Ferguson "It's often reported as hardy, but generally it is
NOT hardy beyond USDA Zone 8, even in drier regions with warmer average
temperatures The species varies in hardiness, with all varieties able to
withstand limited amounts of hard freezing. The problem is that the
plants just can't seem to handle the long winters with repeatedly freezing
nights."

Dave has grown plants from along the coast and from south of the San
Gabriel/San Bernardino axis, all the way down to Catavinia in Baja California.
None of these has been reliably cold hardy in Albuquerque New Mexico, but
all of them were capable of surviving a brief exposure of - 10C (12 F)
in an unheated greenhouse here.

A friend of mine Tim Behan who lives on Rhode
Island has tested Yucca whipplei in the winter of 2000-2001 read his surprising
results at one
of his web pages.

According to Bollinger (1998) is Yucca whipplei hardy in Europe
in protected areas. In Central Europe plants can be grown to flowering
size in only 13 years from seeds.

I have tried to grow the plants in pots inside the unheated greenhouse,
but our winters are too long with too many days and nights of freezing
temperatures. I will however try to grow one of the forms that can be found
in the mountane forest just below the snow line, in the bed inside the
unheated greenhouse, it may survive inhere?

Notes:1. Some authors place Yucca
whipplei and it's subspecies in the Genus Hesperoyucca. One
of the reasons to this is that the seedlings form a distinct bulb which
no other Yucca does. another reason cold also be that Yucca whipplei
and
it's subspecies are pollinated by another species of Yucca moth which only
do feed on Yucca whipplei!

2 In February 2003, I got
a note about Yucca whipplei ssp.
parishii from Carl Wishner
a Biologist from California:

Carl Wishner states that Yucca whipplei ssp. parishii
(Jones) Haines, is also described from the southern front of the San Gabriel
and San Bernardino Mountains. The nominate ssp. whipplei is reported from
Orange and Riverside counties south to northern Baja California. There
are no corresponding pages for these taxa. None of Haines' described subspecies
are recognized in the "Jepson Manual" for California, however, and he continues
to recognize them as distinct.